The specific goals of this study are: a) to investigate whether the cellular tropic and cytopathic characteristics of distinct HIV-1 isolates allows for infection and death of hematopoietic stem/progenitor cells; and b) to investigate whether HIV-1 infection reduces the "stem cell capacity" of the putative CD34++CD38+/- stem cells engrafted NOD/SCID mice transplanted with a human thymus. GCRC Normal Blood Donors (SPID 0075) were essential to this project.